FARM IMPLEMENTS OF IRRIGATION. 179 



duced. It is an iron spiral working in an open masonry 

 inclined trough. The spiral has a diameter of about 

 1 metre, and is worked by iron gearing turned by a bullock. 

 On 0*75 to 1 metre lift this screw will water 1 feddan per 

 hour, say 250 cubic metres, which is far beyond the capa- 

 city of any other bullock machine. It is strongly made 

 and costs complete with masonry about L.E. 60. 



FIG. 57 ft, THK Ar.fHiMF.pEAX Srr:Kw 



Large archimedean screws driven by steam engines have 

 also been in use in Egypt. Those at Khatatbeh pumping 

 station were 4 metres in diameter and 12 metres long, with 

 a capacity of 120 cubic metres per minute each. Faulty 

 construction at this installation rather discredited screw* 

 of this large size in Egypt. 



The benefit of a screw is that, turned fast or slow within 

 reasonable limits, it will give out water in exact proportion 

 to its speed. For this reason if attached to a windmill it 

 would always lift some water, whereas a centrifugal pump 



